The extensive pollution problems in modem industrialized societies are adversly affecting our health and environment. Ever increasing industrialisation and number of automobiles make it absolutely necessary to constantly monitor and control air pollution in the environment. In many industries gases have become increasingly important as raw materials. Therefore, it has become very important to develop highly sensitive gas sensors and systems to prevent accidents and air pollution due to gas leakage. As a result, new and powerful research areas have emerged in our battle for awareness and environmental and health monitoring. The research and development of solid state gas sensor is one such area. Such sensors should allow continuous monitoring of the concentration of particular gases in the environment in a quantitative and selective manner.
Solid state gas sensors use an appropriate material, either in bulk form or in thick or thin film form as gas sensing element. The working principle for gas detection of these sensors is based on change in i) work function; ii) resistance iii) dielectric constant and or iv) mass of the sensing element due to adsorption of a gas. The resultant change in any one of these properties is measured to determine the presence and percentage of the gas in the ambient. Most of conventional sensors employ bulk or thick films of a gas sensing material.
However, in recent sensors, thin films are used as modern thin film preparation techniques give better control on gas sensing properties of a material.
Hitherto known gas sensors based on thin films of materials are of three kinds. First, metal oxides such as SnO.sub.2, ZnO, Ga.sub.2 O.sub.3 etc. (Taguchi: UK Patent 1280809, Mosely: Sensors & Actuators, B 6, 1992). Second, catalytic metals like Pd and Pt (I. Lundstrom et. al: Appl. Phys. Letts. 26, 1975, Sh. Kaihatsu: JP 1213563 A) and third, a special class of organic materials such as Phthalozyanine, Polypyrol u. a. (P. M. Burr et.al: Thin-Solid Films, 151, 1987, M. Josowicz: "Organic semiconductors as chemical sensor materials", Habilitation Universitat der Bundeswehr Munchen). The working principle for gas detection of most of these sensors is based on change in work function of the thin films, due to adsorption of a gas which in turn produces electrical signal. This signal is measured for qualitative and quantitative detection of a particular gas under test in the ambient. However, for the first type of sensors i.e. metal oxide based gas sensors, energy is needed for chemo-physical reactions on the sensing layer. This is achieved by heating the sensitive layer. Therefore, operating temperature of metal oxides based sensors is few hundred degree celsius e.g. from 300.degree. C. to 1000.degree. C. Reference may be made to (i) UK Patent 1280809--A. Mandelis (ii) C. Christofides: A Series of monographs on Analytical Chemistry and its Applications, ed. J. D. Winefordner, Vol. 125, John Wiely & Sons, INC. N.Y. 1993, Chapters 1 to 3 & Refs. there in, and (iii) M. Fleischer et. al. Sensor & Actuators B 25-27, 1995. The sensitivity, selectivity and dynamic response of these metal-oxide based sensors are temperature dependent and this necessitates them to operate at elevated temperatures. To realize these sensors, a heater is provided to heat the sensing film. Generally a thin film heater is fabricated on the back side of the sensing film and electrical power is supplied to it to achieve the desired temperature. Additionally, a temperature sensor is also required to be incorporated with the gas sensor to control the power to regulate the temperature of the sensor. Therefore, a heater and a temperature sensor are integral components of a gas sensing system based on metal-oxide sensors. In this way, heating requirement of the sensor makes fabrication technology and design of the sensing system very complex. It also necessitates thermal isolation between the sensor and the measurement circuit. Further, sensitivity of these sensors is critically dependent on film structure and its preparation technique (Peschke, M. et.al, sensors & Actuators B1, 21, 1990, Peschke, Ph. D. Thesis, 1990, Universitat der Bundeswehr Munchen, A. Mendelis and C. Christofides: Ref. above). The high temperature requirement of these sensors precludes their applications to battery operating equipments. Although application of catalytic metals films like Pt and Pd in FET or CCFET (Capacitive controlled FET) gas sensors has demonstrated room temperature operation (I. Lundstrom et. al, Sensors & Actuators Al, 1981 and Gergintschew, Z., Kornetzky, P., Schipanski, D., Patentschrift DE 433875 At.) but such ammonia gas sensors suffer from a drawback of their cross-sensitivity to hydrogen and hydrocarbon based gases. In addition to poor selectivity, metal film sensors also exhibited ageing effects (K. Dohos et.al Sensors & Actuators 4, 1983). Thus, despite of possibility of room temperature operation, these sensors have not yet became popular due to selectivity problem. For some special applications, the organic materials films are highly sensitive and selective but life time of organic film sensors is limited. Additionally organic materials films are not compatible to micro-electronic fabrication technologies and therefore, are not suitable for large scale production. In brief, the gas sensors known to date either can operate at elevated temperatures or suffer from selectivity problem.